This invention relates generally to closed loop or "Sagnac" fiber optic interferometers, and, more particularly, the invention relates to sensors for environmental quantities which can affect fiber optic waveguides in fiber optic interferometers.
Interferometers work on the principal of phase change between two coherent signals which are interfered together. Closed loop interferometers measure the relative phase change on a "global" scale, i.e., they measure the overall relative phase change between two counterpropagating light signals in a single loop of fiber.
The optical fiber loops of interferometers are sensitive to a large number of environmental effects such as temperature, acoustic pressure, vibration, motion, and electric and magnetic fields. These external phenomena can change the optical transmission characteristics of the fiber such as by changing the birefringence in the fiber or the geometric path length or the velocity of propagation of light signals through the fiber, which can result in changes in the amplitude, phase, or polarization of light propagating therethrough. This sensitivity to environmental effects means that fiber interferometer loops may function as sensor elements so long as the desired quantity to be sensed can be isolated from the other environmental quantities to which the fiber is sensitive.
A distinction should be made in the reader's mind between environmental effects which are "reciprocal" (i.e., effects which do not cause a phase shift between the counterpropagating waves) and environmental effects which are "non-reciprocal" (i.e., those effects which do cause a phase shift between counterpropagating waves). Additionally, reciprocal and non-reciprocal effects may be thought of either as being on a "global scale", wherein the effect of the environmental phenomena is considered with respect to the loop as a whole, or as being on a "differential scale", wherein the phenomena is considered only with respect to a localized, tiny segment of the fiber. Certain environmental effects such as rotation and the Faraday effect cause non-reciprocal effects on the differential scale in that they will cause a relative phase change between counterpropagating light signals travelling through the same tiny piece of fiber. These non-reciprocal differential effects are cumulative so that the resulting effect on a global scale is also non-reciprocal. Other environmental effects, such as pressure, are reciprocal on the differential scale in that they cause no net relative phase change between counterpropagating light signals travelling through the same tiny piece of fiber. On a global scale, the cumulative effect of these reciprocal differential effects is ordinarily also reciprocal. Such reciprocal effects on the differential scale, therefore, cannot be measured by a closed loop interferometer unless some way is devised to make these effects non-reciprocal on the global scale even though they are reciprocal on the differential scale.
In the prior art, the closed loop interferometer is typically used to sense non-reciprocal differential effects, such as rotation. When operated with a polarizer which limits the optical paths of the couterpropagating light to a single one of the polarization modes, as disclosed in copending U.S. patent application Ser. No. 319,311, filed Nov. 9, 1981 which is a continuation in part of U.S. patent application Ser. No. 307,095 filed on Sept. 30, 1981 which is a continuation in part of U.S. patent application Ser. No. 249,714 filed on Mar. 31, 1981, the closed loop interferometer is basically stable and, on a global scale, is relatively insensitive to environmental effects other than non-reciprocal, differential effects, such as motion. In such interferometers, any reciprocal, differential environmental phenomena, such as temperature or pressure, affects the fiber optical light transmission characteristics of both counterpropagating light signals substantially equally and, therefore, results in little or no relative phase change between the counterpropagating signals. The present invention teaches a closed loop interferometer configuration which senses differential reciprocal effects, such as accoustic pressure, by making their effect on the global scale non-reciprocal.